The present invention relates to a method and an apparatus for measuring a beam spot of scanning light, and more particularly to a method and an apparatus capable of continuously measuring and displaying properties of the scanning light such as a shape of the beam spot, which change in a primary scanning direction and a light advancing direction.
The followings are well-known techniques for measuring the properties of the scanning light such as the beam spot shape.
It is on sale “Beam Alyzer” manufactured by the Melles Griot Inc., wherein a knife edge is provided on a rotary drum, and a section profile and a spot size of a beam spot are measured on the basis of optical power variation when an incident light beam crosses the knife edge. It is also on sale “Beam Scan” manufactured by Photon Inc., wherein a focal condition of an incident light beam is similarly measured by a narrow slit provided on a rotary drum.
However, these apparatuses can measure only a stationary light beam. Further, in a case where the spot shape is different between the stationary state and the scanned state, accurate results cannot be obtained. For example, in a case where a dynamic pressure bearing motor is used in a light deflector (a light scanning device), the posture of an axis is actually different in a stationary state and a rotating state. Moreover, a mechanism for accurately positioning a beam in the light receiving portion of a sensor is necessary and a time is required for carrying out a positioning operation.
Japanese Patent Publication No. 53-31147A discloses a scanning optical system in which a light beam is incident on a polygon mirror at an angle which is not perpendicular to a secondary scanning direction. In this optical system, the light beam tends to be twisted.
Japanese Patent Publication No. 64-13514A teaches that a spot size in a primary scanning direction and a secondary scanning direction with a vertical slit extending in the secondary scanning direction and an oblique slit inclined to a direction parallel to a scanning line.
Japanese Patent Publication No. 6-70583B teaches that a major axis diameter, a minor axis diameter and an inclined angle are calculated with three slits including one vertical slit and two oblique slits. It is based on an assumption that the beam spot is elliptical.
Japanese Patent No. 2876650 discloses that an inclined angle and dimensions in a primary scanning direction and a secondary scanning direction of an elliptical beam spot are simultaneously calculated with three slits including a vertical slit extending in the secondary scanning direction and two oblique slits. It is also disclosed that two vertical slits are provided to calculate a scanning velocity which is to be used to calculate the spot size. It is further disclosed a combination of two slits each of which has a vertical edge and an oblique edge.
Japanese Patent No. 3050996 discloses that a spot size in a primary scanning direction and a scanning velocity are measured with a slit extending in a secondary scanning direction. It is also disclosed that a spot size in a secondary scanning direction is measured with two pairs of slits which are slightly inclined from the primary scanning direction. Here, the extending direction of the slits are matched with the scanning direction of the light beam. It is further disclosed that a pair of oblique slits extending in two directions are provided to eliminate effect caused by an error in an angle formed by the scanning line and the slits. The inclined angle of the elliptical spot is out of consideration.
Japanese Patent Publication No. 6-118329A discloses that irregularity of the pitch of the scanning lines is measured with a one-dimensional linear array CCD sensor.
Japanese Patent Publication No. 2000-292308A discloses that fluctuations of the position of the scanning line in a secondary scanning direction is measured with a triangular slit.
Besides, it is well-known that the optical power is measured with a power meter. However, also in this case, a mechanism for accurately positioning a beam in the light receiving portion of a sensor is necessary and a time is required for carrying out a positioning operation.
In an electrophotography, a change in a density is represented by the size of a halftone dot or the thickness of a line in order to carry out gradation recording. In the former case, the dots are arranged regularly in an oblique direction. In the latter case, oblique lines parallel with neither a primary scanning direction nor a secondary scanning direction are used. In a case where multicolor printing is to be carried out, the directions of the dot arrays or the lines are varied color by color.
In that case, if an almost elliptical focal spot (beam spot) is inclined obliquely, the density tends to be increased when an inclined angle approaches to the angle of the dot arrays or the lines, and the density tends to be reduced when the inclined angle is turned in a different direction. When the inclined angle of the focal spot is varied depending on a position in the primary scanning direction, the density or tone is changed depending on a change in the inclined angle. In case of a halftone image in which two colors overlap each other, one of the colors becomes dark and the other color becomes light depending on an inclination of the major axis of the elliptical spot when the directions of screens of two colors (the direction of the dot arrays or the lines) are different from each other. When the inclination in the direction of the major axis of the elliptical spot is varied depending on a position in the primary scanning direction, the color of the halftone image is changed depending on the position. Human eyes are more sensitive to a change in coloring (tone/hue) than a change in the density of a single color. For this reason, it is necessary to continuously evaluate the inclined angle of the major axis of the elliptical spot in the primary scanning direction.
In the prior art described above, the inclined angle of the spot can be measured in a specific position in the primary scanning direction, but continuous evaluation throughout in the primary scanning direction cannot be executed. Even if a plurality of measuring points are provided, long time is required for the measurement. Even if measuring data are quantitatively obtained, since numeral data are merely enumerated, it is hard to instinctively and visually recognize a problematic situation. Furthermore, it is impossible to measure a large number of measuring items once at a time.